Quantum Dots Stabilized With A Metal Thiol Polymer

ABSTRACT

A composition of matter comprises a plurality of quantum dots and a metal thiol polymer that acts to stabilize the quantum dots. In certain embodiments, the metal thiol polymer is a zinc thiol polymer. The zinc thiol polymer may be a zinc alkanethiolate. The zinc alkanethiolate may be zinc dodecanethiolate (Zn-DDT). A composition comprising a plurality of quantum dots and a metal thiol polymer may be formulated with one or more additional polymers as a quantum dot-containing bead or as a quantum dot-containing composite material—e.g., a multilayer film.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. Non-Provisional applicationSer. No. 15/061,753, filed on Mar. 4, 2016, which claims the benefit ofU.S. Provisional Application No. 62/128,354, filed on Mar. 4, 2015.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention generally relates to semiconductor nanoparticles(or “quantum dots”). More particularly, it relates to the use of a metalthiol polymer (e.g., zinc 1-dodecanethiol polymer) to providesemiconductor nanoparticles with enhanced stability.

2. Description of the Related Art Including Information Disclosed Under37 CFR 1.97 and 1.98

There is widespread interest in exploiting the properties of compoundsemiconductors consisting of particles with dimensions on the order of2-50 nm, often referred to as quantum dots (QDs) or nanocrystals. Thesematerials are of commercial interest due to their size-tuneableelectronic properties which can be exploited in many commercialapplications such as optical and electronic devices and otherapplications that range from biological labelling, photovoltaics,catalysis, biological imaging, LEDs, general space lighting andelectro-luminescent displays among many other new and emergingapplications.

The most studied of semiconductor materials have been the chalcogenidesII-VI materials namely ZnS, ZnSe, CdS, CdSe, CdTe; most noticeably CdSedue to its tuneability over the visible region of the spectrum.Reproducible methods for the large scale production of these materialshave been developed from “bottom up” techniques, whereby particles areprepared atom-by-atom, i.e., from molecules to clusters to particles,using “wet” chemical procedures.

Two fundamental factors, both related to the size of the individualsemiconductor nanoparticle, are responsible for their unique properties.The first is the large surface to volume ratio; as a particle becomessmaller, the ratio of the number of surface atoms to those in theinterior increases. This leads to the surface properties playing animportant role in the overall properties of the material. The secondfactor being that, with many materials including semiconductornanoparticles, there is a change in the electronic properties of thematerial with size. Moreover, because of quantum confinement effects,the band gap gradually becomes larger as the size of the particledecreases. This effect is a consequence of the confinement of an“electron in a box” giving rise to discrete energy levels similar tothose observed in atoms and molecules, rather than a continuous band asobserved in the corresponding bulk semiconductor material. Thus, for asemiconductor nanoparticle, because of the physical parameters, theelectron and hole, produced by the absorption of electromagneticradiation (a photon, with energy greater than the first excitonictransition), are closer together than they would be in the correspondingmacrocrystalline material, moreover the Coulombic interaction cannot beneglected. This leads to a narrow bandwidth emission, which is dependentupon the particle size and composition of the nanoparticle material.Thus, quantum dots have higher kinetic energy than the correspondingmacrocrystalline material and consequently the first excitonictransition (band gap) increases in energy with decreasing particlediameter.

Core semiconductor nanoparticles, which consist of a singlesemiconductor material along with an outer organic passivating layer,tend to have relatively low quantum efficiencies due to electron-holerecombination occurring at defects and dangling bonds situated on thenanoparticle surface which can lead to non-radiative electron-holerecombinations. One method to eliminate defects and dangling bonds onthe inorganic surface of the quantum dot is to grow a second inorganicmaterial, having a wider band-gap and small lattice mismatch to that ofthe core material epitaxially on the surface of the core particle, toproduce a “core-shell” particle. Core-shell particles separate anycarriers confined in the core from surface states that would otherwiseact as non-radiative recombination centers. One example is a ZnS shellgrown on the surface of a CdSe core. Another approach is to prepare acore-multi shell structure where the electron-hole pair is completelyconfined to a single shell layer consisting of a few monolayers of aspecific material such as a quantum dot-quantum well structure. Here,the core is of a wide band gap material, followed by a thin shell ofnarrower band gap material, and capped with a further wide band gaplayer, such as CdS/HgS/CdS grown using substitution of Hg for Cd on thesurface of the core nanocrystal to deposit just a few monolayers of HgSwhich is then over grown by a monolayer of CdS. The resulting structuresexhibit clear confinement of photo-excited carriers in the HgS layer. Toadd further stability to quantum dots and help to confine theelectron-hole pair one of the most common approaches is by epitaxiallygrowing a compositionally graded alloy layer on the core; this can helpto alleviate strain that could otherwise led to defects. Moreover, for aCdSe core, in order to improve structural stability and quantum yield, agraded alloy layer of Cd_(1-x)Zn_(x)Se_(1-y)S_(y) can be used ratherthan growing a shell of ZnS directly on the core. This has been found togreatly enhance the photoluminescence emission of the quantum dots.

Doping quantum dots with atomic impurities is an efficient way also ofmanipulating the emission and absorption properties of the nanoparticle.Procedures for doping of wide band gap materials, such as zinc selenideand zinc sulfide, with manganese and copper (ZnSe:Mn or ZnS:Cu), havebeen developed. Doping with different luminescence activators in asemiconducting nanocrystal can tune the photoluminescence andelectroluminescence at energies even lower than the band gap of the bulkmaterial whereas the quantum size effect can tune the excitation energywith the size of the quantum dots without having a significant change inthe energy of the activator-related emission.

The widespread exploitation of quantum dot nanoparticles has beenrestricted by their physical/chemical instability and incompatibilitywith many of the materials and/or processes required to exploit thequantum dots to their full potential, such as incorporation intosolvents, inks, polymers, glasses, metals, electronic materials,electronic devices, bio-molecules and cells. Consequently, a series ofquantum dot surface modification procedures has been employed to renderthe quantum dots more stable and compatible with the materials and/orprocessing requirements of a desired application.

A particularly attractive field of application for quantum dots is inthe development of next generation light-emitting diodes (LEDs). LEDsare becoming increasingly important in modern-day life and it isenvisaged that they have the potential to become one of the majorapplications for quantum dots, for example, in automobile lighting,traffic signals, general lighting, and backlight units (BLUs) for liquidcrystal display (LCD) screens. LED-backlit LCDs are notself-illuminating (unlike pure-LED systems). There are several methodsof backlighting an LCD panel using LEDs, including the use of eitherwhite or RGB (Red, Green, and Blue) LED arrays behind the panel andedge-LED lighting (which uses white LEDs around the inside frame of theTV and a light-diffusion panel to spread the light evenly behind the LCDpanel). Variations in LED backlighting offer different benefits. LEDbacklighting using “white” LEDs produces a broader spectrum sourcefeeding the individual LCD panel filters (similar to cold cathodefluorescent (CCFL) sources), resulting in a more limited display gamutthan RGB LEDs at lower cost. Edge-LED lighting for LCDs allows a thinnerhousing and LED-backlit LCDs have longer life and better energyefficiency than plasma and CCFL televisions. Unlike CCFL backlights,LEDs use no mercury (an environmental pollutant) in their manufacture.Because LEDs can be switched on and off more quickly than CCFLs and canoffer a higher light output, it is possible to achieve very highcontrast ratios. They can produce deep blacks (LEDs off) and highbrightness (LEDs on).

Currently, LED devices are made from inorganic solid-state compoundsemiconductors, such as AlGaAs (red), AlGaInP (orange-yellow-green), andAlGaInN (green-blue). However, using a mixture of the availablesolid-state compound semiconductors, solid-state LEDs which emit whitelight cannot be produced. Moreover, it is difficult to produce “pure”colors by mixing solid-state LEDs of different frequencies. Therefore,currently the main method of color mixing to produce a required color,including white, is to use a combination of phosphorescent materialswhich are placed on top of the solid-state LED whereby the light fromthe LED (the “primary light”) is absorbed by the phosphorescent materialand then re-emitted at a different wavelength (the “secondary light”),i.e., the phosphorescent materials down-convert the primary light to thesecondary light. Moreover, the use of white LEDs produced by phosphordown-conversion leads to lower costs and simpler device fabrication thana combination of solid-state red-green-blue LEDs.

Current phosphorescent materials used in down-converting applicationsabsorb UV or mainly blue light and convert it to longer wavelengths,with most phosphors currently using trivalent rare-earth doped oxides orhalophosphates. White emission can be obtained by blending phosphorswhich emit in the blue, green and red regions with that of a blue orUV-emitting solid-state device. i.e., a blue-light-emitting LED plus agreen phosphor such as, SrGa₂S₄:Eu²⁺, and a red phosphor such as,SrSi:Eu²⁺ or a UV-light-emitting LED plus a yellow phosphor such as,Sr₂P₂O₇:Eu²⁺; Mn²⁺, and a blue-green phosphor. White LEDs can also bemade by combining a blue LED with a yellow phosphor, however, colorcontrol and color rendering is poor when using this methodology due tolack of tunability of the LEDs and the phosphor. Moreover, conventionalLED phosphor technology uses down-converting materials that have poorcolor rendering (i.e., color rendering index (CRI)<75).

Rudimentary quantum dot-based light-emitting devices have been made byembedding colloidally produced quantum dots in an optically clear (orsufficiently transparent) LED encapsulation medium, typically a siliconeor an acrylate, which is then placed on top of a solid-state LED. Theuse of quantum dots potentially has some significant advantages over theuse of the more conventional phosphors, such as the ability to tune theemission wavelength, strong absorption properties and low scattering ifthe quantum dots are mono-dispersed.

For the commercial application of quantum dots in next-generationlight-emitting devices, the quantum dots may be incorporated into theLED encapsulating material while remaining as fully mono-dispersed aspossible and without significant loss of quantum efficiency. The methodsdeveloped to date are problematic, not least because of the nature ofcurrent LED encapsulants. Quantum dots can agglomerate when formulatedinto current LED encapsulants, thereby reducing the optical performanceof the quantum dots. Moreover, even after the quantum dots have beenincorporated into the LED encapsulant, oxygen can still migrate throughthe encapsulant to reach the surfaces of the quantum dots, which canlead to photo-oxidation and, as a result, a drop in quantum yield (QY).

Quantum dots (QDs) may be incorporated into polymer beads for a varietyof reasons. Labeled beads are disclosed in U.S. Pat. Nos. 7,674,844 and7,544,725. Multi-layer-coated quantum dot beads are described in U.S.Pub. No. 2014/0264196 and the preparation of quantum dot beads having asilyl surface shell is described in U.S. Pub. No. 2014/0264193. Quantumdot polymer beads have also been developed for lighting and displayapplications. The incorporation of quantum dots into beads offersbenefits in terms of processing, protecting quantum dots from photooxidation, and ease of color rendering. However, one detrimental effectof incorporating quantum dots into beads is that the quantum yield ofthe quantum dots is often reduced.

Traditionally, the QD beads have been prepared via suspensionpolymerisation by mixing the QDs with (meth)acrylate resins containinglauryl methacrylate (LMA) as a monomer, trimethyloyl propanetrimethacrylate (TMPTM) as a cross-linker and phenylbis(2,4,6 trimethylbenzoyl)phosphine oxide (IRGACURE® 819) as a photoinitiator, and curingusing UV-LED light. Although bright QD beads have been successfullysynthesized by suspension polymerisation, due to the large resultingbead size, those beads are not suitable to use in many applications—e.g.lighting and display applications. The synthesis of small beads in thesize range below 50 microns is very challenging and a drop in thephotoluminescence quantum yield (QY) is usually observed with decreasingbead size.

Recently, a facile method for making both red and green, small (<50 μm),bright, QD beads for lighting and display applications has beendeveloped. The process involves the addition of TWEEN® polysorbatesurfactant [ICI Americas, Inc.] together with an aqueous polyvinylalcohol (PVOH) solution, leading to the formation of smaller, brighterbeads.

The method has been extended to include the use of SPAN® surfactants[Croda International PLC], a series of polysorbitan ester surfactants,in the place of the TWEEN® surfactants.

In such a suspension polymerization, a solution comprised of unwashedmonomer (e.g., lauryl methacrylate), a cross-linker (e.g., trimethyloylpropane methacrylate) and a photoinitiator (e.g. IRGACURE 819) may beadded to dry quantum dots to produce a QD-resin solution. An aqueoussolution of PVOH and a surfactant (e.g., TWEEN 80) may be stirred atabout 400-1000 rpm and the QD-resin solution injected under a nitrogenatmosphere. Optionally, the surfactant may be added to the QD-resinsolution. The solution may be allowed to equilibrate and then cured byexposure to UV light. The resulting small QD polymer beads may berecovered by washing with cold water and acetonitrile and drying undervacuum.

Both red-emitting and green-emitting QD beads have been successfullysynthesized by suspension polymerization, but the quantum yield (QY) ofred beads in particular has heretofore been below that required for mostdisplay applications. Conventional quantum dot polymer beads are alsosensitive to heat, i.e., the quantum yield of the quantum dots issignificantly reduced after the beads have been heated. This imposeslimitations for post-processing of the beads, e.g. atomic layerdeposition (ALD) coating, and may limit their processing into devices,e.g. soldering of an LED containing fluorescent beads onto a circuitboard.

In view of the significant potential for the application of quantum dotsacross such a wide range of applications, including but not limited to,quantum dot-based light-emitting devices such as display backlight units(BLUs), work has already been undertaken by various groups in an effortto develop methods for increasing the stability of quantum dots so as tomake them brighter, more long-lived, and/or less sensitive to varioustypes of processing conditions. For example, reasonably efficientquantum dot-based light-emitting devices can be fabricated underlaboratory conditions building on current published methods. However,there remain significant challenges to the development of quantumdot-based materials and methods for fabricating quantum dot-baseddevices, such as light-emitting devices, on an economically viable scaleand which provide sufficiently high levels of performance to satisfyconsumer demand.

BRIEF SUMMARY OF THE INVENTION

It has been discovered that the addition of a metal thiol coordinationpolymer into quantum dot beads and composites has at least two, distinctbenefits: 1) it significantly increases the QY in comparison to beadsand composites synthesized without a metal thiol polymer; and, 2) beadsand composites made with the inclusion of a metal thiol polymer are morethermally stable, i.e. they retain a higher proportion of their initialQY after being heated.

The present invention comprises a method to prepare heavy-metal-freequantum dot beads and composites using an organometallic zinc sulfidepolymer such as a Zn-DDT (1-dodecanethiol) polymer either as an additivein an otherwise conventional synthesis method or using a metal thiolpolymer as the polymer matrix of the bead or composite.

Previous attempts to make small QD beads by the above-described methodwithout the Zn-DDT polymer resulted in beads with substantially lowerbrightness than beads synthesized with Zn-DDT polymer.

The method of the present invention makes highly bright QD beads andcomposites that retain a greater portion of their QY after being heated.These attributes make a substantial impact on improving the performanceof the QD polymer beads and composites.

Low cross-linked, highly bright beads and composites are suitable formaking films for display applications. Higher cross-linked bright beadsare suitable for ALD-coating, for example, with a metal oxide surfacecoating such as Al₂O₃, to lower the water vapor transmission rate andthe permeability to other gases and liquids. Such coated beads may alsobe suitable for display and lighting applications.

Because the addition of certain types of Zn-DDT (depending on theparticular synthesis route used) into beads and composites reduces thefull width at half maximum (FWHM) of the peak in the emission spectrumand reduces the amount of red shift that is typically observed whenquantum dots are incorporated into a matrix, improved color renderingmay result.

Quantum dots with a thin or incomplete inorganic shell or quantum dotswith very high purity (and therefore lacking a large excess ofstabilizing ligands) show an unacceptable loss of photoluminescencequantum yield when handled. This has been a significant impediment totheir incorporation into applications such as use as a color conversionmaterial in display and lighting applications.

Quantum dot-containing composites also benefit from the addition of ametal-thiol polymer. For example, a two-phase resin system to producequantum dot films for display applications using a liquid-liquid systemof an epoxy outer phase with an acrylate inner phase that are cured toprovide a solid-solid final film exhibited enhanced stability andperformance when a Zn-dodecanethiol polymer was used as a host materialfor the quantum dots in the inner phase of the two-phase resin system.

Various quantum dot-(metal-thiol polymer) composite materials accordingto the invention have been developed and combined with various outerphase type resins and the performance and lifetimes in films evaluated.The use of the metal-thiol polymer as the host material has provedbeneficial for maintaining external quantum efficiency of the film, aswell as edge ingress stability under light stress and dark ingressstability under thermal stress.

It is to be understood that other aspects of the teachings will becomereadily apparent to those skilled in the art from the followingdescription, wherein various embodiments are shown and described by wayof illustration.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

FIG. 1 is a contemplated structure of one particular zinc thiol polymer.

FIG. 2 is a schematic diagram of the synthesis of small QD beads withZn-DDT by a suspension/emulsion polymerization according to theinvention.

FIG. 3A shows the backlight unit (BLU) spectrum and FIG. 3B shows theresulting color gamut simulation for a particular film according to theinvention.

DETAILED DESCRIPTION OF THE INVENTION

As discussed above, quantum dots with a thin or incomplete inorganicshell or quantum dots with sufficiently high purity (those lacking asufficiently large excess of stabilizing ligands) exhibit anunacceptable loss of luminescence intensity over time whenphoto-excited. This has been a significant obstacle to theirincorporation into commercial applications such as use as a colorconversion material in display and lighting applications.

It has been discovered that the addition of a zinc thiol polymer toquantum dot-containing beads or a composite materials unexpectedlyincreases the QY of the quantum dot beads or composite in comparison toquantum dots or composites synthesized without a metal thiol polymer.Moreover, quantum dot composites made with the inclusion of a metalthiol polymer are more thermally stable.

A zinc thiol polymer is relatively easy to make and incorporate intoquantum dots. One particular method of the prior art is described by SuChen, Chunhui Hu, Li Chen and Nanping Xu, in Chem. Commun., 2007,1919-1921.

Very little or no loss in the solution quantum dot quantum yield (QY)was observed when a zinc thiol polymer was included in the preparationof quantum dot beads and composites. Incorporation of the polymeradditive into the QD-containing resin helps to stabilize thephoto-luminescence (PL) QY—i.e., there was less loss of PL QY in beadsand composites having a metal thiol polymer incorporated therein.

The present invention involves the enhancement of quantum dot stabilityto environmental factors and processing conditions by inclusion intoquantum dot-(metal-thiol polymer) beads and composite materials and thesubsequent use of stabilized quantum dots in efficient and stable colordown-conversion applications.

The stability enhancement of Zn-dodecanethiol polymer on quantum dotssuggested that, if that polymer were used as a host material for thequantum dots in the inner phase of a two-phase resin system, abeneficial improvement in stability and performance could be realized.

A quantum dot-(metal-thiol polymer) composite material was developed andcombined with various outer phase type resins and the performance andlifetimes of the quantum dots in films evaluated. The use of themetal-thiol polymer as the host material proved beneficial formaintaining external quantum efficiency (EQE) of the film, as well asedge ingress stability under light stress and dark ingress stabilityunder thermal stress.

The present invention also has application in the use of metal-thiolcoordination polymers as hosts for quantum dots to improve themaintenance of the quantum yield (QY) of the quantum dots duringprocessing into an application format, in an exemplary case (but notlimited to) a film made from an aggressive polar polymer which wouldordinarily be expected to degrade quantum dot performance.

Zn-DDT polymer may be synthesized by a variety of techniques anddifferent alkane chain lengths of the ligands may be used. However, eachsynthesis path produces material that has varying properties. Thesynthesis pathway that produces the most useful material known as of thefiling date hereof is as described below. Ratios of each precursor maybe varied to produce polymer with different properties:

Anhydrous zinc acetate was degassed at 100° C. in THERMINOL® 66 [SOLUTIAINC., 575 MARYVILLE CENTRE DRIVE, ST. LOUIS, MISSOURI 63141], asynthetic heat transfer fluid comprising a modified terphenyl, for 1 hr.The solution was placed under an inert N2 atmosphere and raised to atemperature of 230° C. and annealed for 1 hr. After one hour,dodecanethiol was added in an amount relative to the amount of anhydrouszinc acetate used (Zn-to-S ratio of 1:0.9). The solution was left toanneal for one hour and was then cooled to 70° C. The solution wasprecipitated at 70° C. with a volume of acetone equal to the amount ofTHERMINOL 66 used.

The Zn-DDT polymer precipitated out as a thick, waxy dilatant. Thismaterial showed shear thickening properties reminiscent of corn starchin water. The produced material was washed with acetone and thendissolved into toluene at 90° C. for 30 minutes. The toluene solutionproduced was colorless and did not precipitate upon cooling to roomtemperature. Not all of the material was soluble in toluene. Theinsoluble material was then dried using a vacuum and subsequently groundinto a fine powder.

For the material that was soluble in toluene, the toluene solution wasprecipitated with two volume equivalents of acetone. The precipitatedmaterial was then centrifuged and sonicated in acetone in order toremove toluene. Toluene seemed to have an affinity to this material. Theprecipitate was then dissolved into cyclohexane and re-precipitated withacetone. This final precipitate was then dried under vacuum and groundusing a pestle and mortar. The drying and crushing process was repeatedthree times, producing a free-flowing, white solid.

Both the insoluble and soluble part were analyzed and tested in beads.In general, the insoluble fraction contains crystalline material anddispenses well in conventional bead resins (e.g., LMA/TMPTM). Thesoluble Zn-DDT fraction may be useful in other applications which arenot detailed here.

An alternative synthetic pathway, using the precursor zincdimethacrylate was developed to change the functionality of the Zn(DDT)polymer:

Anhydrous zinc dimethacrylate was degassed at 100° C. in THERM INOL 66for 1 hr. The solution was then placed under an inert atmosphere (N₂)and the temperature was raised to 230° C. and annealed for 1 hr. After 1hour, dodecanethiol was added relative to the amount of anhydrous zincdimethacrylate used (Zn-to-S ratio of 1:0.9). The solution was left toanneal for 1 hr. and was then cooled to 70° C.

The solution was precipitated at 70° C. with a volume of acetone equalto the amount of THERM INOL 66 used. This produced a very fineprecipitate which was completely insoluble in any solvent previouslymentioned. The precipitate was further washed with acetone to purify theend product. The washed solid was then dried under vacuum for 24 hrs. toproduce a very fine, free-flowing, off-white powder.

It is contemplated that other alkanethiols including functionalizedalkanethiols may have application in the present invention. For example,the DDT may have a reactive functional group on the end of the alkanechain opposite the thiol group (or, in the case of branched alkanes, ata position remote from the thiol group) which reactive functional groupallows cross linking or bonding into the polymer host itself.

The application of the present invention is illustrated below in twoexemplars—QD beads, and QD-containing composite materials:

Quantum Dot Beads

The synthetic procedure of making QD beads by suspension polymerizationis illustrated schematically in FIG. 2.

Example 1: Preparation of Low Cross-Linked Small, Bright Red Beads UsingZn-DDT Polymer

Preparation of PVOH Solution: Aqueous polyvinyl alcohol (PVOH) solutionwas prepared by dissolving PVOH in deionized water and stirringovernight. Prior to bead synthesis, the PVOH solution was filtered toremove any undissolved PVOH or dust particles. 1% TWEEN® 80 surfactant[CRODA AMERICAS LLC, 1209 ORANGE STREET, WILMINGTON DELAWARE 19801] wasadded to the 4% aqueous PVOH solution.

Preparation of Zn-DDT Resin Solution: To prepare the resin, redInPZnS-based alloyed quantum dots (13 mg/3 mL of resin) [PLmax=627 nm,FWHM=56 nm, QY (dilute, toluene)=80%] dissolved in toluene weretransferred to an amber glass vial containing a stirrer bar, under aninert atmosphere. The toluene was removed under reduced pressure andcontinuous stirring. Once all visible traces of solvent were removed,the residue was heated to 40° C., for 45 minutes, under vacuum to removeany residual solvent. A stock solution was prepared by adding degassedlauryl methacrylate (LMA, 5.2 mL) and trimethyloyl propane methacrylatecross-linker (TMPTM, 0.8 mL) to the photoinitiator mixture IRGACURE® 819initiator (40 mg, 0.74 wt. %) and IRGACURE 651 (40 mg, 0.74%) [BASF SECOMPANY, CARL-BOSCH-STR. 38 LUDWIGSHAFEN GERMANY 67056]. The mixture wasstirred in the dark to ensure complete dissolution. The required amountof the stock solution was then added to the dry QD residue, under aninert atmosphere and dark conditions, to form a resin solution. Theresin solution was stirred overnight to ensure complete dispersion ofthe QDs. The next day, the required amount of Zn-DDT powder was degassedfor 1 hr. then the QD resin solution was added into it and stirredovernight.

Preparation of QD Beads with Zn-DDT: In a 20 mL glass vial, PVOH/Tween80solution (4/1 wt. %, 10 mL) was degassed for a few hours under avacuum/nitrogen cycle. For each experiment, 1 g resin solution wasinjected into an aqueous solution of PVOH/Tween80, under continuousstirring at 800 rpm, under N₂. The solution was allowed to equilibratefor 15 minutes, then cured for 10 minutes under a UV LED kit to form QDbeads. The QD beads were subsequently washed with water andacetonitrile, then dried under vacuum. Characterization data ispresented in Table 1, along with the QY after heating in vacuum for over48 hrs.

TABLE 1 Low cross linked beads, 1/0.154 LMA/TMPTM, as typically used inbead films. Sample Zn-DDT QY % after ID Concentration % QY % PL FWHMheating at 80° C. B361 0 48 639 58 27 B362 2.5 60 640 55 n/a B363 5 62639 53 45 B364 7.5 67 637 54 n/a B365 10 69 634 53 58 B380 12.5 74 63654 n/a B381 15 73 637 54 58

The data in Table 1 illustrates that the QY of the beads increased withincreasing the Zn-DDT concentration. Additionally, the FWHM was reducedand the amount of red shift of the dots in the beads was reduced. Thedata in Table 1 also illustrates that beads made with more than about10% of Zn-DDT polymer retain a significantly higher proportion of theinitial QY on heating.

As shown in FIG. 3, the incorporation of Zn-DDT polymer into beadsoccurs as beads with higher Zn-DDT loading (sample IDs B364-365) settledon the bottom of the vial. Whereas beads with no or less Zn-DDT float(sample IDs B361 and B362). Interestingly, it has been found that beadswith 5% Zn-DDT will make a stable colloidal latex solution.

Example 2: Preparation of Higher Cross-Linked Bright Red Beads UsingZn-DDT Polymer

A similar procedure to Example 1 was followed, except using TWEEN® 80surfactant (3 wt. %) [CRODA AMERICAS LLC, 1209 ORANGE STREET, WILMINGTONDELAWARE 19801], a 1:0.5 ratio of LMA:TMPTM and 1 wt. % of IRGACURE 819for the resin solution preparation. 3 mL of the Zn-DDT QD resin solutionwas injected into a deoxygenated PVOH solution in a 250 mL tornedoreaction vessel. The equilibrium time was 5 minutes and the curing time10 minutes at 800 RPM. The characterization data is shown in Table Y.

TABLE 2 Details of higher cross-linked beads before and after ALDcoating with Al₂O₃. Zn-DDT Initial QY concentration QY after ALDAfter:before Bead ID (%) (%) (%) ratio B199 41 0 15 0.37 B200 55 10 440.80 B201 62 20 56 0.90

The data in Table 2 demonstrates that the QY of higher cross-linkedbeads has increased by increasing the Zn-DDT concentration. Table 2 alsoillustrates that the QY of the beads with 20% Zn-DDT retained about 90%of the initial QY after ALD coating with Al₂O₃. For the beads with noZn-DDT only 37% of the initial QY was retained after ALD coating.

QD-Containing Composite Materials

Preparation of a quantum dot-(metal-thiol polymer) composite materialmay be achieved by mixing the two components, quantum dots andmetal-thiol polymer, as toluene solutions followed by removal of thetoluene by vacuum distillation. Additionally, a scattering agent (to aidblue light absorption and light extraction) may be added. An example ofthe preparation of a composite using Zn-dodecanethiol polymer is givenin Example 3. In this case, the metal-thiol polymer used wasZn-dodecanethiol polymer which was prepared following a literaturemethod (S. Chen, C. Hu, L. Chen and N. Xu, Chem. Commun., 2007, 1919).Post preparation, the photo-luminescent (PL) optical properties of theground powders were investigated by fluorescence spectroscopy using aHamamatsu spectrometer equipped with an integrating sphere accessory.The PL peak wavelength, full width at half maximum (FWHM) of theemission peak and the photo-luminescent quantum yield (PLQY) weredetermined using an excitation wavelength of 450 nm. Example data ispresented in Table 3.

The PLQY performance of the ground composite powders compares favorablyto other solid forms such as polymerized beads for, example, where PLQYmeasurements are usually lower than 70% due to damage induced by theaction of the free radical polymerization and the aggressive outer-phasematerial needed during the bead synthesis.

TABLE 3 Example photo-luminescent PL, FWHM and PLQY data for quantumdot-(metal-thiol polymer) composite material samples. QD CompositeMaterial Dilute Toluene QD Specs QD Loading PLQY Measurements Sample PLFWHM PLQY (mg/g polymer) PL FWHM PLQY % Abs 1101+ (<35 μm) 528 45 77 35535 45 74 20% 1129 (>35 μm) 522 45 80 35 529 44 71 21% 1116 (>35 μm) 52444 79 35 533 43 68 22% 1116 (<35 μm) 524 44 79 35 528 45 72 25% 1101+(>35 μm) 528 45 77 35 536 44 74 20% 1129 (<35 μm) 522 45 80 35 527 47 7417% 1130 (<35 μm) 634 53 83 8 642 57 85 19% 258A (<35 μm) 631 51 78 8635 55 68 19% 1130 (>35 μm) 634 53 83 14 642 57 78 18% 1182 (<35 μm) 52543 78 35 530 44 63 11% 1183 (<35 μm) 628 52 81 14 631 53 78 11%

To test the utility of these powders, a number of LEDs were preparedusing small, low-power LEDs in a standard SMD 3528 package. The methodof preparation for these is detailed in Example 4. After preparation,the external quantum efficiency (EQE) of the composites in the LEDs wasdetermined by comparison with blank LEDs (i.e., LEDs without QDs)prepared using the same Optocast 3553 UV-cure epoxy resin without anycomposite material present and operating at a forward current of 20 mA.Given the very high absorbances of the composites in the LED, the EQEsare quite good and much better than could be achieved with dotsdispersed directly into Optocast UV and/or heat-cured epoxy alonewherein they undergo significant quenching due to the negativeinteraction with the polar epoxy.

TABLE 4 Example photo-luminescent PL, FWHM and PLQY data for quantumdot-(metal-thiol polymer) composite material samples incorporated intolow-power, SMD 3528, LEDs. Dilute Toluene QD Specs QD Loading CuredOptocast LEDs Sample ID PL FWHM PLQY (mg/g polymer) PL FWHM EQE % Abs1101+ (<35 μm) 528 45 77 35 544 42 43 84% 1129 (>35 μm) 522 45 80 35 54140 55 81% 1116 (>35 μm) 524 44 79 35 540 42 59 70% 1116 (<35 μm) 524 4479 35 536 42 54 64% 1101+ (>35 μm) 528 45 77 35 542 42 50 75% 1129 (<35μm) 522 45 80 35 538 41 48 79% 1130 (<35 μm) 634 53 83 8 646 56 41 72%258A (<35 μm) 631 51 78 8 641 54 32 78% 1130 (>35 μm) 634 53 83 14 64456 57 60% 1182 (<35 μm) 525 43 78 35 538 40 57 73% 1183 (<35 μm) 628 5281 14 635 54 65 47%

To further test the utility of the prepared composite powders, they weredispersed into various polar resins and luminescent films were preparedby sandwiching the resin between two pieces of barrier film and curingthe resin by UV light exposure. An example of the preparation of theresin mixture and subsequent film preparation is presented in Example 5.Following preparation, the films were cut into pieces approximately14×18 mm for EQE and lifetime stress test measurements. A number ofpolar resins were investigated and the composite materials yielded filmswith relatively high EQEs in nearly all cases. Details of resins andamounts of composite materials along with film EQEs are presented inTable 5. Across a range of samples and resins, the lowest EQE achievedwas 41% with most resins yielding films with EQEs of 49%.

TABLE 5 Composite-resin formulation details and resulting film EQEs.Mass Mass Green Red Mass Film QDs, QDs, Mass Resin, Code mg mg BaSO₄Resin mg EQE 403B 100 60 65 Epotek OG142 740 48% 404 100 60 65 EpotekOG142 500 48% 407A 100 60 65 Custom acrylate 1 500 49% 407B 100 60 65Custom acrylate 1 500 49% 411A 110 41 65 Custom acrylate 1 500 49% 411B110 41 65 Custom acrylate 2 500 41% 412D 110 52 65 Custom acrylate 2/500 47% Custom acrylate 1 (75/25 mix) 416A 110 52 65 Custom acrylate 3600 45% 416B 110 52 65 Custom acrylate 4 600 50% 416C 110 52 65 Optocast3553 600 46% 418A 172 98 73 Custom acrylate 3 1600 52% 425A 520 300 0Custom acrylate 5 2400 47% 425B 541 315 0 Custom acrylate 5 2400 48%425C 568 340 0 Custom acrylate 5 2400 49%

Some of the resins used were commercially available. However, some wereprepared as custom formulations. The different resin formulations cangenerally be described as:

-   -   custom acrylate 1: Epoxy acrylate resin with bi-functional        monomer    -   custom acrylate 2: Low glass transition temperature (T_(g))        acrylate resin    -   custom acrylate 3: Epoxy acrylate resin with bi-functional        monomer    -   custom acrylate 4: High T_(g) acrylate resin    -   custom acrylate 5: Epoxy acrylate resin with bi-functional        monomer

Further illustration of the impact of the quantum dot-(metal-thiolpolymer) composite material on performance maintenance can be seen inTable where it is clear that the EQEs of films using a solidQD-(metal-thiol polymer) composite is much improved compared to thoseusing a QD in a liquid monomer inner phase. In particular, customacrylate 2, which produces a low T_(g) acrylate, is clearly damaging tothe liquid monomer inner-phase performance producing a low EQE of 13%compared to around 40% when a more benign resin (like custom acrylate 1)is used. This is most likely due to the metal-thiol polymers ability toform super hydrophobic structures [S. Chen, C. Hu, L. Chen and N. Xu,Chem. Commun., 2007, 1919] which is very good at protecting the quantumdots from deleterious components in the outer phase resin.

TABLE 6 Comparison of film EQEs when dots are incorporated in metalthiol polymer composites or in a liquid monomer inner phase in the resinprior to curing. Inner Phase Outer Phase Film EQE liquid LMA/TMPTMCustom acrylate 2 13% QD-(metal-thiol polymer) composite solid Customacrylate 2 41% liquid LMA/TMPTM Custom acrylate 1 38% QD-(metal-thiolpolymer) composite solid Custom acrylate 1 49%

Further evidence of how the metal-thiol polymer helps to protect thequantum dots is provided by heat-treating the samples. Table 7 showsPLQY measurements of samples before and after heat treatment andindicates that the composites maintain PLQY performance very wellcompared to other types of solids with embedded quantum dots, in thiscase, acrylate beads.

TABLE 7 Comparison of QY maintenance after heat treatment of acrylatebeads and Zn-thiol polymer composite solids. Initial PLQY afterAfter:before Bead PLQY heat treatment ratio B199 acrylate polymer beads41% 15% 0.37 QD-Zn/DDT polymer composite 79% 73% 0.92

To test the usability of the composites in liquid crystal display (LCD)BLUs as a color conversion material, the QD-(metal-thiol polymer)composite materials were dispersed into different resins andsandwich-type films were made wherein a QD-containing layer issandwiched between two pieces of barrier film. Following fabrication,the films were tested on a blue BLU for performance in terms ofluminance and white point and using liquid crystal module (LCM)transmission spectra for the red, green and blue pixels possible colorgamut performance simulated. An example spectrum of a QD-(metal-thiolpolymer) composite film and subsequent color gamut are reproduced inFIG. 3A and FIG. 3B, respectively. In the case of this sample film(425B), the luminance on a 550-nit BLU was 2280 nits and utilized adiffuser/BEF/DBEF optical film stack. The color gamut simulation wasdone using transmission spectra determined from a commercially availabletelevision receiver and, when applied to the BLU data, gave a largecolor gamut with overlap of 96% of the Digital Cinema Initiatives P3(DCI-P3) standard. The BLU white point (x,y) was 0.267, 0.238 which gavea desired white point of 0.290, 0.296 after transmission spectra wereapplied.

Further to BLU performance data, the films were also cut into smallerpieces (approximately 19 mm×14 mm) and subjected to a number ofdifferent lifetime stress tests designed to test the maintenance ofperformance of the film under accelerated use conditions. In thereal-world application the film would ordinarily be exposed toapproximately 3-10 mW/cm² of blue light irradiance within the BLU. Assuch, the films were stress tested with irradiances and humidities of2.4 mW/cm² at room temperature (real time test) and 106 mW/cm² at 60°C./90% relative humidity (accelerated test). Additionally, they weresubjected to a dark test at 60° C./90% relative humidity (RH).

In all tests, the films maintained good performance, evidencing nodegradation in performance in the real-time test over the course of 2500hrs. of testing. At 60° C./90% RH conditions, the films also showedrelatively good maintenance of performance, maintaining over 80%relative performance when exposed to 106 mW/cm² blue light stress andover 90% for green-emitting quantum dots and over 70% for red-emittingquantum dots when stressed in the dark. Importantly, in the dark test,there was no visual sign of edge ingress which has proven to be aproblem in other systems.

Example 3. Preparation of Quantum Dot-(Metal-Thiol Polymer) CompositeMaterial

Two milliliters of a toluene solution of green-emitting quantum dots(heavy metal-free semiconductor nanoparticles) at a concentration of 35mg/mL were added to 14 mL of glacial acetic acid. The resultingprecipitate was isolated by centrifugation and the supernatantdiscarded. The solid collected was rinsed with acetonitrile and thendissolved in n-hexane and centrifuged. Any remaining undissolved solidswere discarded and the hexane solution was transferred to a separatingfunnel. Acetonitrile (15 mL) was added and the funnel shaken. Afterphase separation, the polar layer was discarded. This extraction processwas repeated twice more to yield a coating on the separating funnelglass and two colorless liquid phases, which were discarded. The coatingwas rinsed once more with acetonitrile. If the washings were acidic, thecoating was rinsed again until the rinse solvent tested pH ˜neutral.Finally, the coating was dissolved in toluene (3 mL).

Separately, 1 g of Zn-dodecanethiol polymer was dissolved in toluene (10mL) and mixed with 1 mL of the above prepared quantum dot solution.Barium sulfate powder (70 mg) was added and the mixture sonicated. Theresulting suspension was evaporated to dryness in vacuo to yield ayellow solid that was placed under nitrogen. Traces of solvent were thenremoved using a freeze drying technique. The sample was cooled withliquid nitrogen, placed under reduced pressure for a several minutes,brought slowly back to room temperature (where it was left under reducedpressure for several minutes), then backfilled with a nitrogenatmosphere. The cycle was repeated at least 5 times until a free flowingsolid was obtained. The sample was then transferred to aninert-atmosphere glove box where it was ground in an agate pestle andmortar and sieved through a 35-micron sieve to yield a free-flowing,highly luminescent powder.

Example 4. Preparation of Test LEDs

Ground and sieved QD-(Zn-dodecanethiol polymer)-BaSO₄ composite material(9 mg) was weighed into a clean glass vial. UV curing epoxy Optocast3553 (120 μL) was added and the suspension thoroughly mixed. A smallvolume (3 μL) of the suspension was deposited into the empty well of a3528 LED package and the resin cured by exposure to UV light (360 nm,170 mW/cm², 64 seconds).

Example 5. Preparation of Luminescent Composite Films

Sample 1116 (172 mg), sample 1130 (98 mg) and BaSO4 were weighed into aclean amber vial in an inert-atmosphere glove box. Anepoxy-acrylate-based resin (800 mg) was added and the mixture mixed wellwith a clean metal spatula until an even-colored viscous mixture wasobtained. The vial was then capped and removed from the glove box and,under yellow light, dispensed onto the barrier side of a PET-basedbarrier film with an oxygen transmission rate (OTR) of approximately10⁻² g/m²/day. Another piece of barrier film was placed on the top ofthe resin with the barrier side towards the resin and the sandwichpulled through a mangle to spread the resin and define the filmthickness. Following this step, the film was placed in a UV curing ovenfor 30 seconds where a dose of approximately 800 mJ was delivered andthe film fully cured.

Advantages

The use of metal-thiol coordination polymers as hosts for quantum dotsin composite materials has a number of advantages. First, there is astrong interaction between the polymer and the quantum dots such thatthe quantum dots are very stable when interacting with the polymer andthe polymer shields the quantum dots from deleterious outside agentssuch as initiators and polar outer phase components. Second, thecomposite can be prepared as a solid so that subsequent color renderingof films is relatively straightforward and only involves the weighing ofdifferent powders followed by mixing with the outer phase resin. Third,different colored solid composites will not mix when mixed into theouter phase resin, keeping color rendering simple. Finally, becausethere is no curing step and the quantum dots are simply embedded in thehost matrix there are no losses of performance resulting from damage tothe quantum dots by curing.

The foregoing presents particular embodiments of a system embodying theprinciples of the invention. Those skilled in the art will be able todevise alternatives and variations which, even if not explicitlydisclosed herein, embody those principles and are thus within the scopeof the invention. Although particular embodiments of the presentinvention have been shown and described, they are not intended to limitwhat this patent covers. Those skilled in the art will understand thatvarious changes and modifications may be made without departing from thescope of the present invention as literally and equivalently covered bythe following claims.

What is claimed is:
 1. A method for preparing quantum dot beadscomprising: preparing a first solution comprising a monomer, across-linker, and a polymerization photoinitiator; dispersing quantumdots in the first solution to produce a second solution; mixing thesecond solution with zinc thiol polymer to produce a third solution;adding the third solution to an aqueous solution of polyvinyl alcoholand a surfactant under continuous stirring to form a fourth solution;and exposing the fourth solution to ultraviolet light to form quantumdot beads.
 2. The method of claim 1, wherein the zinc thiol polymer is azinc alkanethiolate.
 3. The method of claim 2, wherein the zincalkanethiolate is zinc dodecanethiolate (Zn-DDT).
 4. Quantum dot beadsprepared according to the method of claim
 1. 5. A method for preparingzinc dodecanethiolate (Zn-DDT) polymer comprising: adding a zinc salt toa synthetic heat transfer fluid to form a first solution; annealing thefirst solution under an inert atmosphere; adding dodecanethiol to thefirst solution in an amount relative to the amount of the anhydrous zincsalt used to provide a second solution; annealing the second solution;cooling the second solution; and adding a non-solvent to precipitateZn-DDT polymer.
 6. The method of claim 5, further comprising: dissolvingthe precipitated Zn-DDT in a non-polar solvent to produce a thirdsolution and insoluble material; and drying the insoluble material. 7.The method of claim 5, further comprising: drying the precipitatedZn-DDT polymer to produce a powder consisting essentially of Zn-DDTpolymer.
 8. The method of claim 5, wherein adding dodecanethiol to thefirst solution in an amount relative to the amount of anhydrous zincsalt used to provide a second solution comprises adding dodecanethiol inan amount such that the second solution has a Zn-to-S ratio of about1:0.9.
 9. The method of claim 5, wherein the zinc salt is zinc acetate.10. The method of claim 5, wherein the zinc salt is zinc dimethacrylate.11. A method for preparing quantum dot beads comprising: preparing afirst solution comprising a monomer, a cross-linker, and apolymerization photoinitiator; dispersing quantum dots in the firstsolution to produce a second solution; mixing the second solution with aZn-DDT polymer produced according to the method of claim 5 to produce athird solution; adding the third solution to an aqueous solution ofpolyvinyl alcohol and a surfactant under continuous stirring to form afourth solution; and exposing the fourth solution to ultraviolet lightto form quantum dot beads.
 12. A composite material comprising: quantumdots embedded in a resin comprising a Zn-DDT polymer produced accordingto the method of claim 5, wherein the quantum dots embedded in the resincomprising the Zn-DDT polymer are in the form of a ground compositepowder.
 13. A composition of matter comprising: a plurality of quantumdots; and a Zn-DDT polymer produced according to the method of claim 5.14. The composition recited in claim 13, further comprising a resinmaterial different from the Zn-DDT polymer.
 15. The composition recitedin claim 13 formulated as a dry powder.
 16. The composition recited inclaim 13 formulated as a solution.